Optical fibre technology requires characterization and control of various fibre properties during the process of manufacturing of the fibre from a preform.
Fibre performance in general depends on the geometric uniformity and the dimensions of the core and cladding layers of the optical fibre. The outer diameter is generally measured at a point shortly after the fibre is formed during the drawing process, typically immediately below the neck-down region.
U.S. Pat. No. 4,280,827 describes a fibre diameter measurement circuit including a source, a detector that senses the presence of interference fringes, wherein the detector signal is connected to two signal comparing means via a respective delay circuit connected to the source. The outputs from the signal comparing means are combined and counted in order to generate a succession of counts representative of the diameter of successive axial portions of the advancing fibre.
To obtain high-quality fibres, structural defects, such as holes or bubbles, should be minimised or avoided. Holes or bubbles typically occur at the centre of the fibre, although they can be located anywhere in the fibre cross-section.
U.S. Pat. No. 5,185,636 describes a method and an apparatus for detecting defects in optical fibres based on different parameters from those used to measure fibre diameter so that the steps of diameter control and defect detection can be uncoupled in the overall detection system. The disclosed techniques are based on effects on the far-field interference pattern produced by holes.
In U.S. Pat. No. 6,313,909 a scattered light signal is filtered and the resulting signal is compared to a defect detection threshold to determine the presence of defect-related components in the scattered light signal.
U.S. Pat. No. 5,406,374 describes a method for inspecting a rod-like optical fiber preform for the presence of bubbles and/or inclusions comprising photographing images scattered by the bubbles/inclusions through a side face of the preform by a video camera while light rays are incident upon the whole end face of the preform from a white lamp and discriminating and detecting the bubbles/inclusions through image-analysis of image signals of the photographed images. Light rays are incident upon the optical fiber preform and photograph of the images of the preform by the video camera are made while rotating the optical fiber preform around the longitudinal axis of the optical fiber preform.
A synthetic quartz drawing apparatus provided with a detector for bubbles is disclosed in JP 10167744 A.
A testing device is disclosed in WO 2011/052541, having a detector detecting the forward scattering light intensity distribution of a preform, while rotating the preform by a rotating mechanism. A determination unit determines from the light intensity distribution whether through-holes are formed at predetermined positions.
Various technologies for optical fiber production may lead to the formation of holes or bubbles trapped in the fibre. For example, core preforms formed by Outside Vapor Deposition (OVD) are obtained by deposition of silica and doped silica soot on a target. At the end of the deposition process, the target is removed resulting in a soot preform with a central hole. The soot preform is consolidated and then stretched so that the central hole is collapsed thereby forming a glass core rod having a surface region with low or no dopant content.
In a known process, described in US 2003/0140658, after creating a vacuum in the central hole, the consolidated core preform is placed in a vertical furnace in which the fusion of a lower end of the preform is carried out. The fusion of the lower end causes the walls of the hole to collapse because of the vacuum created in the hole. The fused glass material cools, forming an elongated cylindrical element of a predetermined diameter, which is stretched downwards by a traction device. The disclosed apparatus for applying traction to an elongated element is provided with an opto-electronic sensor placed in the proximity of the upper portion of the elongated cylindrical element for measuring the diameter.
The collapse of the hole during stretching can be incomplete and/or take place with the generation of defects. Core preforms made by Modified Chemical Vapour Deposition (MCVD), by Plasma Chemical Vapour Deposition (PCVD) processes or by other inside vapour deposition processes may exhibit a similar problem.
A final glass preform to be drawn into an optical fibre can be produced by depositing cladding glass soot onto the core rod, for example by an OVD process, followed by consolidation.
Optical micrometers for determining the position and the dimension of an object of circular cross-section are known.
U.S. Pat. No. 4,492,473 discloses an opto-electronic measuring device utilising a rotating mirror to produce a scanning beam which is then collimated, directed at an object to be measured, and the light then collected and a digital pulse train derived therefrom representing the position of the object, by utilizing a shaft encoder which is driven by the same motor which rotates the mirror for the generation of pulse signals which are proportional to the angular velocity of the encoder and the mirror.
U.S. Pat. No. 4,991,308 concerns a diameter gauge for measuring the outside diameter of a part having a circular cross-section. The gauge comprises two arms converging to form a V-shaped channel, a light source positioned along one side of the channel and, opposite the light source, along the other side of the channel, a light detecting device.
In U.S. Pat. No. 5,175,595, a non-contact measuring device is described, in which light generated from a source of light that is allowed to enter a polygonal mirror rotating at a constant speed via a lens. The light that is reflected by the polygonal mirror is then allowed to enter a collimator lens to thereby turn the light into a scanning light movable in parallel to the optical axis of the collimator lens and to allow the light to be focused at the position of an object to be measured.
U.S. Pat. No. 6,278,520 relates to an optical micrometer for measuring the maximum diameter of work pieces, e.g. a tapered cutting tool, along the length of the work piece. A laser micrometer for measuring a work piece, which includes: means for generating a laser beam path such that a portion of said beam path passes by a part of the work piece; means for receiving and processing said portion of said beam path to determine size of said first part; a V-shaped seat having a first side surface and a second side surface, wherein said surface form a seat to receive the work piece, and wherein the V-shaped seat seats the work piece to permit at least a portion of the work piece to be disposed within said beam path; and apparatus for rotating said work piece to cause slidable rotation of said work piece in said V-shaped seat such that said work piece is maintained in contact with said two sides while seated therein by said apparatus.